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resulting from of differing lip lengths, or an offset chisel point, causing the drill to ‘spiral-down’ through the workpiece, as it progresses through the part (see Fig. 70). ‘Bell-mouthing’ of the drilled hole is attributable to the chisel point and is produced by the line-of-contact, as the drill point initially touches the component’s surface, causing it to ‘walk’ until the feed/penetration stabilises itself at the outer corners (i.e. margins) entering the workpiece, whereupon, these mar-gins guide the drill into the part. ‘Bell-mouthing effect’ is produced by the drill chisel point’s eccentric behaviour as it attempts to centralise its rotational motion as it enters, or exit’s the workpiece. ‘Jobber drills’ are considered to be ‘standardised drills’ that are normally utilised for most drilling general operations, un-less otherwise specified. ‘Rigidity rule’: a drill, reamer, tap, or a milling cutter held in a spindle will have its rigidity decreased by the ‘square of the distance’ , namely, if a drill is twice as long it is four times less rigid.NB A cantilevered tool such as a boring bar has its rigidity de-creased by the ‘cube’ or the distance – meaning that too much tool overhang, will seriously reduce tooling rigidity.R = ratio of the transverse reaction at the drill point,T = thrust force,I = system’s ‘moment of inertia’ ,k = √T/E I.As suggested above, this ‘axis slope error’ is initiated when the chisel edge begins to penetrate the workpiece and unless the feed is discontinued, or in some man-ner the error is corrected, the magnitude of deflection will increase as drill penetration continues. The drill’s magnitude of deflection can reach up to 60 µm, under exaggerated drilling conditions. The geometry of the point has been the subject of considerable research and development for many years, with some unusual departures from the ‘stan-dard’ 118° drill point included angle. Typical of these extreme approaches were the so-called ‘Volvo point’ , having a negative 185° included angle – primarily utilised to avoid ‘frittering’ of drilled holes, or the highly positive geometries such as 80° included an-gle used for drilling some plastics. Not only can the point angle be modified, but the shape and profile of the chisel point, or web offers numerously-ground opportunities for detailed geometric modifications, with only some of which being shown in Fig. 47. Four of the most commonly-ground drill point geometries being:• Conventional – the ‘original’ Morse geometry, hav-ing a straight chisel edge, with poor self-centring drilling action (Fig. 46a),• Split-point – there are a range of point-splitting techniques available to alter the point profile, which has the effect of modifying the chisel point to allow a reasonable self-centring action (Fig. 47b), ‘Frittering’ refers to the break-out at the hole’s edge as the drill exit’s the part, on some brittle materials, such as on several Powder Metallurgy compacts. ‘Web’ refers to the internal core of the drill – which imparts mechanical strength to the drill.
The web increases in thick-ness the further one gets from the chisel edge (i.e. shown in Fig. 47 – in lower diagrams and with cross-sections). Hence, if the drill is reground many times, the chisel point width will obviously increase, this necessitates that the chisel point must be ‘thinned’ , otherwise too high a thrust force occurs and an inefficient drilling action will result. ‘Split-point’ ground drills are sometimes referred to as ‘Multi-facet drills’. • Web-thinning – as its name implies, the chisel point is web-thinned/notched, by regrinding to reduce the width of the chisel point, while slightly modify-ing the profile, giving a partial self-centring action (Fig. 47c),• Helical – the chisel point is ground to an ‘S-shape’ , which modifies both the chisel point and its pro-filed shape, improving the drilling performance and self-centring action (Fig.47d).NB On some drills a sophisticated grinding action has imparted drills without a chisel point, which sig-nificantly improves their drill penetration rates into the workpiece, but requires a complex drill regrinding operation to re-sharpen them when the edge becomes ‘dulled’.Not only are drills supplied with appropriate point geometries, but for twist drills the twin spiral flutes of the drill can also be specified – from the tooling manufacturer, as this gives the drill its ‘equivalent of the rake angle’ as found on a single-point turning tool. On conventional jobber drills, the normal flute angle is 29° – giving a relatively ‘slow’ helix (Fig. 47a) and in the past, typically being utilised for drilling most plain carbon steel grades. Conversely, a drill with a ‘quick’ helix angle (Fig. 47d), might be employed to drill soft materials such as certain plastics. Brittle materials on the other hand, which might be utilised typically when drilling Cartridge Brass (i.e. 70Cu 30Zn composition), require a zero, or slightly negative helix. NB It is possible to temporarily modify the drill’s he-lix angle by re-grinding, termed ‘drill dubbing’ , which refers to lightly ‘flash-grinding’ the flutes at the lips to decrease the effective flute helix angle.The main strength of a drill is via its web, or its cross-section which can be changed and as a result, will modify the flute’s geometric profile (i.e. see Fig. 48). In general, drill cross-section are classified in three groups, namely:• Axe-shaped – having well-defined margins (Fig. 48 –top),• Rounded heel – with increased web, but small mar-gins (Fig. 48 – middle),• Rhombic – incorporating a large web, with wide margins (Fig. 48 – bottom).NB Some twist drills feature oil/coolant holes to allow cutting fluid to reach right down toward the cutting edges of the drill, increasing both tool life and improv-ing the hole’s ‘Surface Integrity’ .3.1.3 The Dynamics of Twist Drilling HolesInt roduc t ionThe term ‘drilling’ refers to all production techniques for the manufacture of cylindrical holes in workpieces using chip-making cutting tools, for short-hole 0 and deep-hole drilling operations. The expression ‘solid drilling’ has been introduced in recent years – which is hole-making generation undertaken in a single opera-tion, to differentiate it from that of the previous tech-niques of either: